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SDA

SCL

Start Condition

S

Stop Condition

P

SDA

SCL

Data Line

Stable;

Data Valid

Change

of Data

Allowed

PCA9555

WITH INTERRUPT OUTPUT AND CONFIGURATION REGISTERS

SCPS131D – AUGUST 2005 – REVISED OCTOBER 2006

connected to a positive supply via a pullup resistor when connected to the output stages of a device. Data

transfer may be initiated only when the bus is not busy.

the SDA input/output while the SCL input is high (see Figure 1). After the Start condition, the device address

byte is sent, MSB first, including the data direction bit (R/W). This device does not respond to the general call

address.

After receiving the valid address byte, this device responds with an ACK, a low on the SDA input/output during

the high of the ACK-related clock pulse. The address inputs (A0–A2) of the slave device must not be changed

between the Start and Stop conditions.

stable during the high pulse of the clock period, as changes in the data line at this time are interpreted as control

commands (Start or Stop) (see Figure 2).

A Stop condition, a low-to-high transition on the SDA input/output while the SCL input is high, is sent by the

master (see Figure 1).

Any number of data bytes can be transferred from the transmitter to the receiver between the Start and the Stop

conditions. Each byte of eight bits is followed by one ACK bit. The transmitter must release the SDA line before

the receiver can send an ACK bit. The device that acknowledges must pull down the SDA line during the ACK

clock pulse so that the SDA line is stable low during the high pulse of the ACK-related clock period (see

Figure 3). When a slave receiver is addressed, it must generate an ACK after each byte is received. Similarly,

the master must generate an ACK after each byte that it receives from the slave transmitter. Setup and hold

times must be met to ensure proper operation.

A master receiver signals an end of data to the slave transmitter by not generating an acknowledge (NACK)

after the last byte has been clocked out of the slave. This is done by the master receiver by holding the SDA line

high. In this event, the transmitter must release the data line to enable the master to generate a Stop condition.

Figure 1. Definition of Start and Stop Conditions

Figure 2. Bit Transfer

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